Abstract/Summary

Extensive morphological and physiological adjustments are assumed to underpin the adaptations of diving birds to
high thermoregulatory costs. However, the role of behavioural adaptations has received little consideration. We have assessed
the relative importance of physiological and behavioural adjustments in aquatic endotherms by studying the case of the poorly
insulated great cormorant (Phalacrocorax carbo) in two contrasting thermal environments: Normandy (water temperature
12°C) and Greenland (water temperature 5°C). Major differences were found in the feeding behaviour of birds breeding in the
two regions. Greenland birds showed a 70% reduction in time spent swimming relative to those in Normandy. Reduction in
Greenland was achieved first by reducing time spent on the surface between dives and secondly by returning to land in between
intensive bouts of diving. Total daily energy intake of cormorants was similar in both areas but prey capture rates in Greenland
were 150% higher than those in Normandy. Our study shows that in a cold foraging environment, poorly insulated great
cormorants significantly increase their foraging efficiency. To do this they rely on ecological adaptive patterns (minimization
of time spent swimming in cold water and increased prey capture rates) far more than physiological adaptations (minimizing
instantaneous costs). This finding supports predictions by Grémillet and Wilson (1999) that great cormorants can cope with
a wide range of abiotic parameters despite their morphological handicaps, provided they can adjust their distribution to exploit
dense prey patches.